Oxidation of phosphorus with steam



Oct. 7, 1952 J. F. SHULTZ 2,613,135

OXIDATION OF PHOSPHORUS WITH STEAM Filed Sept. 28, 1950 l 3 VAPORIZED E SUPERHEATED STEAM PHOSPHORUS 2\ CATALYTIC CONVERTER HYDROGEN p Va 1512? cowo l v mlom TO TREATMENT 0R use AND 5 RECOVERY SYSTEM PHOSPHORIC ACID JOHN F SHULTZ INVENTOR.

Patented Oct. 7, 1952 UNITED STATES" 2,613,135 OXIDATION oFPHosPnoaUs WITH STEAM John F. Shultz, Pittsburgh,PaUassigfiorto Tennessee Valley Authority, a corporation of the United States .ATENT OFFICE processes which have been proposed fall into two I general classes. In the first of these, the reaction is carried out in liquid phase under pressure and at relatively low temperatures, as is shown by U. S. Patent 1,848,295, Ipatiew. In the second class, phosphorus is reacted with steam in gas phase at relatively low pressure and at relatively high temperatures, as is shown by U. S. Patent 1,605,960, Liljenroth and Larsson The first class, as typified by Ipatiew, has the advantage that relatively low temperature is used and that the-hydrogen is generated under pressure. This type operation, however, has a number of disadvantages which tend to nullify the advantages and makes the process unsuitable for commercial use. In operations of this type, the reactant must be pumped in the form of an emulsion into the reaction chamber against high pressure, an operation which involves many difficulties. The proportion of water required to give a stable emulsion results in the production of weak phosphoric acid, frequently as low as 50 per cent H3PO'4, which is too dilute for many purposes. And, finally, operation in liquid phase results in very severe corrosion of equipment and in conversion of part of the reacting white phosphorus to the solid red form, which causes .plug

C.-in the presence of catalysts. Catalysts for this amended April 30, 1928; 37-0 OJG. 757) reaction which have been mentioned in patents are the non-alkaline metals of the first group of the periodic system; all the metals of the sixthjto eighth groups, and their phosphides; metallic orthophosphates; ferric oxides; i'errosilicon; various silicates; activated carbon; and others.

Attempts have been made to operate the gasphase process on a commercial or semic'ommercial scale, both in this country and in Europe. Apparently these attempts were unsuccessful, since no commercial plant is inoperation at present, so far as is now known. The results of Work reported by Britzke and Pestov (Trans. Sci. Inst. Fert. (Moscow) 59., pp. 5-160, 1929) indicate that the trouble has been both incompleteness of reaction and contamination of the desired product by side products of the phose phbrris-s'team reaction. The principal contaminants inthe product are phosphine in the hydro gen produced and phosphorous acid in the phosphoric acid. Pho's'phine is very undesirable if the hydrogen is to be used in ammonia synthesis, because phosphine is a poison to ammonia synthesis catalysts. Phosphine is also highly poisonous to workmen who may be exposed to its fumes.

Apparently no prior investigators have been successful in developing a practical catalyst for the phosphorus-steam reaction, or a catalytic method that has all the necessary requirements for a commercial process. To be practical commercially, a catalyst for. this process must not only give substantially complete oxidation of the .phosphorus but must also have physical stability to give long catalyst life without loss of activity and must have chemical stability to prevent loss of catalyst from the reaction zone. 7 The vapor-phase oxidation of phosphorus with steam was attempted at the Tennessee Valley Authoritys plant at Muscle Shoals. A number of catalysts shown in the prior art were tested as to their emci'ency in this reaction. It was found that silver metal, as disclosed by Liljenroth in U. S. Patent 1,605,960, had high initial catalytic activity, but that it was attacked by the reaction products and was rapidly removed from the reaction chamber. found to occur when copper or platinum was used. Copper silicate was tested and was found to have-high catalytic activity, but was also found This same disadvantage was to disintegrate rapidly in the reaction chamber. Cobalt phosphide was tested and was found to be lacking in sufficient catalytic activity to cause complete oxidation of phosphorus.

None of these catalysts were found completely satisfactory for commercial use.

It is an object of this invention to provide a method for a vapor-phase oxidation of phosphorus with steam which is adapted to commercial use.

Another object is to provide such a method which yields as product a complex vaporous mixture in which substantially all phosphorus is in pentavalent and trivalent states.

Another object is to provide a catalytic method for the oxidation of phosphorus with steam in which the catalyst is not removed from the reac tion zone at a rate so great as to make operation impractical.

Other objects and features of novelty will be either specifically pointed out or will become apparent upon reference to the following description, claims, and drawings which describe and show an illustrative embodiment of this invention.

I have now found that commercially practical catalysts may be obtained by supporting a material selected from the group consisting of copper, platinum, their phosphides, and mixtures thereof, on a material selected from the group consisting of pyrophosphates of titanium and zirconium.

I have found that a mixture of phosphorus vapor and steam in such proportions that the mol ratio of water to phosphorus is in the range from about 16:1 to 35:1 passed over such catalyst at a temperature in the range from about 600 to 900 C. at a rate of flow in the range from about 500 to 7500 volumes per volume of catalyst per hour y elds a vanorous mixture in which substantially all phosphorus is oxidized into pentavalent and trivalent states. I have also found that operation in this manner and with these catalysts does not result in removal of the catalyst from the reaction zone at rates so high as to make such operation impractical.

The accompanying drawing is a schematic diagram illustrating a process embodying my invention.

With reference thereto, vaporized phosphorus from a suitable source (not shown) enters via line I and is led to catalytic converter 2. An inert carrier gas may be admixed with this vaporized phosphorus if desired. Water vapor, preferably in the form of superheated steam from any suitable source (not shown) is led into the system via line 3. Line 3 may be arranged to connect with line I outside converter 2, if desired, or may enter converter 2 directly as is illustrated, since for operation of my process it is necessary merely to commingle the vapors of phosphorus and water in a manner to secure a fairly homogeneous mixture. The rate of introduction of vapor is controlled so that the ratio of water vapor to phosphorus vapor in the resulting mixture is in the range from about 16:1 to 35:1, or preferably in the range from about 20:1 to 25: 1.

The mixture of vapors in converter 2 then is passed over and in intimate contact with a catalyst comprising a material selected from the group consisting of copper, platinum, their phosphides, and mixtures thereof, supported on a material selected from the group consisting of pyrophosphates of zirconium and titanium. A method of 4 preparing such catalyst is described subsequently herein.

The degree of pressure maintained in converter 2 is not very critical, but low pressures or substantially atmospheric pressure is preferred because equipment for such pressures can be constructed more cheaply. The temperature of catalyst and reactant in converter 2 may be maintained in the range from about 600 to 900 C. The reaction occurs over this entire temperature range, but with many arrangements of beds of catalyst there is likely to be some accumulation of liquid acids of phosphorus on the catalyst when temperatures below 650 C. are used. I prefer to operate in the temperature range from about 650 to 900 C. in order to avoid any possibility of accumulation of liquid acid on the catalyst, and, for economy in operation, I prefer to operate in the range from about 650 to 700 C.

The mixture of vapors is passed over the catalyst at a rate of flow in the range from about 500 to 7500 volumes per volume of catalyst per hour. I have found that the phosphorous acid content of the phosphoric acid produced increases somewhat With the rate of flow. Therefore, I prefer to operate at rates of flow in the range from about 500 to 2000 volumes of vapor per volume of catalyst per hour when it is desired to produce phosphoric acid containing minimum amounts of phosphorous acid. In this range the phosphorous acid content of the phosphoric acid produced will be minimized.

Reaction products are withdrawn from converter 2 via valved line l and are led to an acid condensation and recovery system 5. Here the products are cooled and partially condensed and are separated by the use of suitable conventional methods and apparatus for separating condensable vapors from noncondensable gases. This results in a liquid fraction of phosphoric acid, containing definite quantities of phosphorous acid, and a noncondensable gaseous fraction of hydrogen, containing small amounts of phosphine. These are withdrawn to storage or use as desired. Alternatively, the mixture of reaction products may be withdrawn via valved line 6 to further treatment or use as desired.

The catalyst of my invention may be prepared by heating an oxide or suitable salt of titanium or zirconium with phosphoric acid in excess of the stoichiometrical amount required for the formation of the corresponding pyrophosphates at about 900 C. The excess acid is necessary in order to avoid the formation of any orthophosphate, which turns to powder and is therefore unsuitable as a catalyst support. These pyrophosphates are hard, nonhygroscopic solids and have small catalytic effect in themselves. Any other methods of forming the pyrophosphates may be used if desired.

The pyrophosphates may then be soaked in a solution of an easily decomposable compound of copper or platinum, such as copper nitrate or chloroplatinic acid. The pyrophosphate then is dried and the copper or platinum compound is decomposed by any suitable method, such as heating or reduction in a stream Of hydrogen. The completed catalyst may be used in the form of a metal supported on the pyrophosphates, or

if desired it may be phosphided by exposing the catalyst comprising the reduced metal supported on pyrophosphates to vapors of phosphorus at high temperature. The exact method of preparing the catalyst is not critical. Any of the known aerosols methods depositing. ametal on-hatalytic supportsmay be used.

.I have found that copper and platinum and their: phosphides are retained verytenaciously by erred by loss of metal, merely by placing a row blocks of the metal in the stream of reaction mixture at ornear the entrance to the catalyst bed. The metal is rapidly taken vupiromsuch blocks into the reaction mixture-and is carried into 'the catalyst bed whereuit is adsorbed onto such supports and results in regeneration of the catalysts.

The following examples illustrate the advantages gained by the useof some of my catalysts and methods as compared to othercatalysts for this reaction.

Example I Phosphorus vapor for introduction into the reaction chamber was generated in a saturator maintained'at a constant temperature in .a molten wax bath. A carrier gas was passed through the saturator at a measured rate, where it picked up phosphorus vapor and carried it through the heated connecting tubes into the reaction chamber. Steam was generated'by 'heating'water in an electrically heated boiler in which steam in an outer Jacket maintained the water lathe/inner compartment oi the boiler at the boiling point. Electrical energy was supplied to a heating coil in the inner boiler to convert water at its boilin point into steam. Theamount of steam produced was calculated on the basis of the electrical energy supplied'to the heating coil.

The phosphorus-steam mixture was passed through a tube filled with catalyst heated to the desired temperature. The reaction products were passed first through a refrigerated trap to collect the acid and any unconvertedphosphorus, and then through an analysis train to determine the amounts'of hydrogen and pho'sphine.

A catalyst was prepared by soaking pumice in ooppernitrate solution and heatingthe product. The resulting copper oxide was reduced by heating in hydrogen at-450 C. Atest with this catalyst under the conditions described above gave the following results.

t1; orictiized o pen avaemgerature, U Space 1 H EO:TP4 lent and 51125:- O. velocity mo 2 ratio trsigazgesr'it average "percent Volume of reactant mixture per volume of catalyst per hour.

The phosphorus oxidation was not sufil'ciently complete in these testsandinaddition, the copper was rapidly removed from the reactionzone by the reaction products. The proportion of phosphorus oxidized by this catalyst of the "prior art may be compared with that obtained in my novel process, used in the following example.

'6 :E'rramplell Zirconium pyrophosphate "was prepared by heating 'zirconium oxide with excess phosphoric acid (over that required to'formpyrophosphate) at 900 C. This was treated with copper-nitrate as described above to give a supported catalyst containing only 0.5 per cent copper. Operating as described .in Example I,the'following results were obtained with this catalyst.

.PJ oxidized v 'topenta- Tempcm- Space 1,021 'valent and tore, O. velocity moleratio trivalent 3 states, percent 700' 500' 25.351 $959 JOB 1-, 090 28. 0:1 99. 3 '800 500 24.111 99.5 700 501) 29:1:1 99.7

Not only was the oxidation practically complete, but thecatalyst showed no.physicaldeterioration over a long series of tests. Therate of copper loss was very'low as compared to catalysts supported on othermaterials or to catalysts of the unsupported-type.

Example III A catalyst was prepared by soaking aluminum silicate gel in chloroplatinic acid solution, drying at C., heating at 580 C. to decompose the chloroplatinic acid, and phosphiding theplatinum by treatment with red phosphorus. This catalyst was not very effective as is shown by the following results.

' 'P oxidized l y to penta- Tempet'a- Space I (B1051 valenta'nd .ture,0. velocity mole ratio trivalent states, percent The catalyst disintegrated during the tests and was removed from the reaction zone. These results may be compared with those obtained by the use .of my method, in the following example.

Example IV P oxidized to'ponta- Tempera- Space "Hm-2P4 valent and ture, C. velocity mole ratio trivalent states,

percent 7 00 500 .23. 6:1 to. -5 700 500 29. 7 1 to. 2 700 501) 35. 1:1 198:5 700 500 37.311 97:8

7 There was some loss of copper from this catalyst in the reaction zone, but not nearly so much as was experienced with massive copper and copper supported on other supports.

Itwas noted that the results obtained using a small amount of copper, as in Example II, were almost as good as the results obtained using 14 per cent of copper in Example IV. Apparently, the small quantity of copper in contact with the surface of my novel catalyst support is retained so tenaciously that it is as effective as larger quantities which, by building up a certain thickness of copper on the surface of the catalyst, act more like massive copper.

Example V Similar experiments have shown that platinum and platinum phosphide similarly supported have catalytic activity equal or superior to that of copper on the same supporting materials. Such catalysts under the conditions given above give a very high degree of oxidation of phosphorus. In addition, platinum and platinum phosphide are removed from the reaction zone by the reaction products at a rate which is considerably slower than that at which copper is removed. However, the high initial and replacement costs of catalysts comprising platinum or platinum phosphide supported on titanium or zirconium pyrophosphates as compared to the cost of copper supported on the same materials are such as to make the copper catalysts described above more economical for most commercial uses.

I claim as my invention:

1. A process for the oxidation of phosphorus with steam which comprises mixing superheated steam and phosphorusvapor'in proportions in the range from about 16:1 to 35 :1; passing the resulting mixture of vapors over and in intimate contact with a catalyst comprising a material selected from the group consisting of copper, platinum, their phosphides, and mixtures thereof supported on a material selected fromthe group consisting of pyrophosphates of titanium and zirconium in a reaction zone; maintaining the temperature of the catalyst and vapors in contact therewith in the range from about 600 to 900 C.; maintaining the rate of flow of said vapors over said catalyst in the range from about 500 to 7500 volumes of vapor per volume of catalyst per hour; and withdrawing a vaporous mixture of reaction products, in which substantially all phosphorus is present in pentavalent and trivalent states, from said reaction zone.

2. A process for the oxidation of phosphorus with steam which comprises mixing superheated steam and phosphorus vapor in proportions in the range from about 16:1 to 35:1; passing the resulting mixture of vapors over and in intimate contact with a catalyst comprising a material selected from the group consisting of copper, platinum, their phosphides, and mixtures thereof supported on a material selected from the group consisting of pyrophosphates of titanium and zirconium in a reaction zone; maintaining the temperature of the catalyst and vapors in contact therewith in the range from about 650 to 900 0.; maintainin the rate of flow of said vapors over said catalyst in the range from about 500 to 7500 Volumes of vapor per volume of catalyst per hour; and withdrawing a vaporous mixture of reaction products, in which substantially all phosphorus is present in pentavalent and trivalent states, from said reaction zone.

3. A process for the oxidationof phosphorus with steam which comprises mixing superheated steam and phosphorus vapor in proportions in the range from about 16:1 to 35:1; passing the resulting mixture of vapors over and in intimate contact with a catalyst comprising a material sele ted from the group consisting of copper, platinum, their phosphides, and mixtures thereof supported on a material selected from the group consisting of pyrophosphates of titanium and zirconium in a reaction zone; maintaining the temperature of the catalyst and vapors in contact therewith in the range from about 650 to 900 0.; maintaining the rate of flow of said vapors over said catalyst in the range from about 500 to 2000 volumes of vapor per volume of catalyst per hour: and withdrawing a vaporous mix ture of reaction products, in which substantially all pho phorus is present in pentavalent and trivalent states, from said reaction zone.

4. A process for the oxidation of phosphorus with steam which comprises mixing superheated steam and phosphorus vapor in proportions in the range from about 20:1 to 25:1; passing the resulting mixture of vapors over and in intimate contact with a catalyst comprising a material selected from the group consisting of copper, platinum, their phosphides, and mixtures thereof supported on a material selected from the group consisting of pyrophosphates of titanium and zirconium in a reaction zone; maintaining the temperature of the catalyst and vapors in contact therewith in the range from about 650 to 700 C.; maintaining the rate of flow of said vapors over said catalyst in the range from about 500 to 2000 volumes of vapor per volume of catalyst per hour; and withdrawing a vaporous mixture of reaction products, in which substantially all phosphorus is present in pentavalent and trivalent states, from said reaction zone.

5. A process for the oxidation of phosphorus with steam which comprises mixing superheated steam and phosphorus vapor in proportions in the range from about 16:1 to 35:1; passing the resultin mixture of vapors over and in intimate contact with a catalyst comprising a material selected from the group consisting of copper, platmum, their phosphides, and mixtures thereof supported on zirconium pyrophosphate in a reaction zone; maintaining the temperature of the catalyst and vapors in contact therewith in the range from about 600 to 900 C.; maintaining the rate of flow of said vapors over said catalyst in the range from about 500 to 7500 volumes of vapor per volume of catalyst per hour; and withdrawing a vaporous mixture of reaction products, in which substantially all phosphorus is present in pentavalent and trivalent states, from said reaction zone.

6. A process for the production of phosphoric acid and hydrogen which comprises mixing superheated steam and phosphorus vapor in proportions in the range from about 16:1 to 35:1; passing the resulting mixture of vapors over and in intimate contact with a catalyst comprising copper supported on zirconium pyrophosphate in a reaction zone; maintaining the temperature of the catalyst and vapors in contact therewith in the range from about 650 to 900 0.; maintaining the rate of flow of said vapors over said catalyst in the range from about 500 to 2000 volumes of vapor per volume of catalyst per hour; anddwizhdrawing a vaporous mixture of reaction pro no 5, in which substantiall is present in pentavalent y an phosphorus from saidreaction zone.

and trivalent states,

7. A process for the production of phosphoric acid and hydrogen which comprises mixing superheated steam and phosphorus vapor in proportions in the range from about 16:1 to 35:1; passing the resulting mixture of vapors over and in intimate contact with a catalyst comprising copper supported on zirconium pyrophosphate in a reaction zone; maintaining the temperature of the catalyst and vapors in contact therewith in the range from about 600 to 900 C. maintaining the rate of fiow of said vapors over said catalyst in the range from about 500 to 7500 volumes of vapor per volume of catalyst per hour; withdrawing a vaporous mixture of reaction products, in which substantially all phosphorus is present in pentavalent and trivalent states, from said reaction zone; and cooling and partially condensing said mixture and separatin a resulting liquid phase composed essentially of phosphoric acid containing a minor proportion of phosphorous acid from a gaseous phase composed essentially of hydrogen containing a'minor proportion of phosphine.

, JOHN F. SHULTZ.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 7 

1. A PROCESS FOR THE OXIDATION OF PHOSPHORUS WITH STEAM WHICH COMPRISES MIXING SUPERHEATED STEAM AND PHOSPHORUS VAPOR IN PROPORTIONS IN THE RANGE FROM ABOUT 16:1 TO 35:1; PASSING THE RESULTIN MIXTURE OF VAPORS OVER AND IN INTIMATE CONTACT WITH A CATALYST COMPRISING A MATERIAL SELECTED FROM THE GROUP CONSISTING OF COPPER, PLATINUM, THEIR PHOSPHIDES, AND MIXTURES THEREOF SUPPORTED ON A MATERIAL SELECTED FROM THE GROUP CONSISTING OF PYROPHOSPHATES OF TITANIUM AND ZIRCONIUM IN A REACTION ZONE; MAINTAINING THE TEMPERATURE OF THE CATALYST AND VAPROS IN CONTACT THEREWITH IN THE RANGE FROM ABOUT 600* TO 900* C.; MAINTAINING THE RATE OF FLOW OF SAID VAPROS OVER SAID CATALYST IN THE RANGE FROM ABOVE 500 TO 7500 VOLUMES OF VAPORS PER VOLUME OF CATALYST PER HOUR; AND WITHDRAWING A VAPOROUS MIXTURE OF REACTION PRODUCTS IN WHICH SUBSTANTIALLY ALL PHOSPHORUS IS PRESENT IN PENTAVALENT AND TRIVALENT STATES, FROM SAID REACTION ZONE. 